1. Field of the Invention
The present invention describes a method and an arrangement for coding transform coefficients in picture and/or video coders and decoders and a corresponding computer program and a corresponding computer-readable storage medium, which can particularly be employed as a novel efficient method for binary-arithmetic coding transform coefficients in the field of video coding (CABAC in H.264/AVC cf. [1]).
2. Description of the Related Art
In present hybrid block-based standards for video coding, such as, for example, MPEG-2 [2], H.263 [3] and MPEG-4 [4], the blocks of quantized transform coefficients (levels) are mapped by a defined scan process to a vector which is coded to code words having a variable length by using a run-length coding and subsequent mapping.
In MPEG-2 [2], the code words having a variable length are associated to two-dimensional events (RUN, LEVEL), wherein LEVEL represents the quantized value of a (significant) transform coefficient not quantized to zero; the run-length RUN indicates the number of subsequent (non-significant) transform coefficients quantized to zero, which, in the vector of transform coefficients, are immediately in front of the present significant transform coefficients. In addition, code words having a variable length are defined for the two special events EOB and ESCAPE. While the EOB event indicates that there are no further significant transform coefficients in the block, the ESCAPE event signals that the existing event (RUN, LEVEL) cannot be represented by the defined alphabet of code words having a variable length. In this case, the symbols RUN and LEVEL are coded by code words of a fixed length.
In the newer coding standards H.263 [3] and MPEG-4 [4], the association of code words having a variable length takes place on the basis of three-dimensional events (LAST, RUN, LEVEL), wherein the binary symbol LAST indicates whether the present significant transform coefficient is the last significant coefficient within the block or whether further significant transform coefficients follow. By using these three-dimensional events, no additional EOB event is required; an ESCAPE event is used in analogy to MPEG-2, wherein the binary symbol LAST is coded additionally to RUN and LEVEL.
The coding of the transform coefficients realized in MPEG-2, H.263 and MPEG-4 has the following disadvantages:                To each coding event only a code word having an integer length can be associated, an efficient coding of events with probabilities larger than 0.5 cannot take place.        The usage of a fixed table for mapping the coding events to the code word having a variable length for all the transform coefficients within a block does not consider symbol statistics depending on position or frequency.        Adaptation to the actually existing symbol statistics is not possible.        No usage is made of inter-symbol redundancies present.        
Annex E of the H.263 standard specifies an optional non-adaptive arithmetic coding in which different predetermined model probability distributions are used,                one each for the first, second and third event (LAST, RUN, LEVEL)/ESCAPE        another one for all the following events (LAST, RUN, LEVEL)/ESCAPE of a block of transform coefficients,        as well as one each for the symbols LAST, RUN and LEVEL, which are coded after an ESCAPE event.        
For the following reasons no appreciable increase of the coding efficiency is, however, possible by this optional arithmetic coding:                The advantage of arithmetic coding that a code word having a non-integer length can be associated to a coding event hardly has any effect on the coding efficiency by using combined events of the form (LAST, RUN, LEVEL).        The advantage of using different probability distributions is eliminated by the fact that adaptation to the actually present symbol statistics is not possible.        
One of the first published methods for coding transform coefficients by an adaptive binary arithmetic coding in a hybrid video coder ensuring the adaptation of probabilities to the symbol statistics present has been presented in [5].
In H.264/AVC [1], a context-adaptive method on the basis of code words having a variable length for coding transform coefficients is specified as the standard method for entropy coding. Here, the coding of a block of transform coefficients is determined by the following characteristics:                Both the number of the significant coefficients within a block and the number of subsequent coefficients quantized to one at the end of the vector of transform coefficients are determined by a symbol COEFF_TOKEN. Depending on the block type and the symbols COEFF_TOKEN already coded/decoded for neighboring blocks, one of five defined code word tables is chosen for coding.        While for the transform coefficients quantized to one at the end of the coefficient vector only a single bit is transferred for specifying the sign, the coding of the values (levels) of the remaining significant transform coefficients takes place in a reverse scan order by means of a combined prefix suffix code word.        If the number of significant transform coefficients is smaller than the number of transform coefficients for the corresponding block, a symbol TOTAL_ZEROS will be coded, which indicates the number of transform coefficients quantized to zero which, in the coefficient vector, are in front of the last significant coefficient. For this, 18 code word tables have been specified, which are switched depending on the number of significant coefficients and the block type.        
The run-length of the (non-significant) coefficients quantized to zero (RUN) in front of a significant coefficient is coded for each significant transform coefficient in a reverse scan order as long as the sum of RUNs already coded is smaller than TOTAL_ZEROS. Depending on TOTAL_ZEROS and the RUNs already coded/decoded, switching between seven code word tables takes place.
Although this so-called CAVLC method (CAVLC: context-adaptive variable length coding), by context based switching the code word tables, allows a considerably more efficient coding of the transform coefficients than the methods specified in MPEG-2, H.263 and MPEG-4, it basically has the following disadvantages:                Switching between different code word tables takes place depending on symbols already coded/decoded, the code word tables, however, cannot be adjusted to the actual symbol statistics.        By using code words having a variable length, events having symbol probabilities larger than 0.5 cannot be coded efficiently. This limitation especially prevents coding symbols having a smaller value range, by means of which a construction of suitable contexts might be possible for switching between different model probability distributions.        
A possible solution for avoiding the disadvantages illustrated of well-known methods for coding transform coefficients in block-based picture and video coders is a combination of an adaptive arithmetic coding and a suitable context generation for using the inter-symbol redundancies. Since the more complicated computing of the arithmetic coding, compared to coding by means of code words having a variable length, is a disadvantage, the possibility of an efficient hardware and software implementation must be considered particularly.